WO1998051614A1 - A modified magnesium hydroxide slurry for use in treating wastewater and a process for producing thereof - Google Patents

A modified magnesium hydroxide slurry for use in treating wastewater and a process for producing thereof Download PDF

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Publication number
WO1998051614A1
WO1998051614A1 PCT/US1998/006477 US9806477W WO9851614A1 WO 1998051614 A1 WO1998051614 A1 WO 1998051614A1 US 9806477 W US9806477 W US 9806477W WO 9851614 A1 WO9851614 A1 WO 9851614A1
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Prior art keywords
magnesium hydroxide
slurry
hydroxide slurry
modified magnesium
weight
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Application number
PCT/US1998/006477
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French (fr)
Inventor
Aileen Reyes Gibson
Mark Thomas Wajer
Original Assignee
Martin Marietta Magnesia Specialties, Inc.
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Application filed by Martin Marietta Magnesia Specialties, Inc. filed Critical Martin Marietta Magnesia Specialties, Inc.
Priority to AU73575/98A priority Critical patent/AU7357598A/en
Publication of WO1998051614A1 publication Critical patent/WO1998051614A1/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F11/00—Compounds of calcium, strontium, or barium
    • C01F11/02—Oxides or hydroxides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F5/00—Compounds of magnesium
    • C01F5/14—Magnesium hydroxide
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F5/00—Compounds of magnesium
    • C01F5/14—Magnesium hydroxide
    • C01F5/16—Magnesium hydroxide by treating magnesia, e.g. calcined dolomite, with water or solutions of salts not containing magnesium
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer

Definitions

  • the invention relates to processes for preparing a modified magnesium hydroxide slurry to be used for treating wastewater, and more particularly to processes for treating wastewater using a modified magnesium hydroxide slurry.
  • the invention encompasses the use of a stable modified magnesium hydroxide slurry containing high calcium hydroxide content that achieves 1 ) effective metal removal to meet discharge limits, 2) significant sludge volume reduction when compared to the amount of sludge generated by lime and caustic soda, particularly in sulfate systems, and 3) increased rate of neutralization compared to standard magnesium hydroxide slurry.
  • Use of the modified magnesium hydroxide slurry according to the invention to treat wastewater is advantageous over the use of, for example, lime, caustic soda and standard magnesium hydroxide slurry.
  • the modified magnesium hydroxide slurry of the invention is advantageous over lime and caustic soda in that it produces less sludge volume and sludge having a higher density and solids content, which makes filtration easier.
  • the modified magnesium hydroxide slurry of the invention buffers to a lower pH.
  • the modified magnesium hydroxide slurry of the invention is advantageous over standard magnesium hydroxide slurry in that it achieves a higher endpoint pH, provides more efficient metal removal, requires less slurry by weight and significantly reduces the reaction time.
  • Magnesium hydroxide in slurry form is useful as a pumpable source of magnesium hydroxide for various chemical processes, including but not limited to the following: (1) pH adjustment, including waste acid and acidic wastewater neutralization; (2) wastewater treatment, including precipitation of heavy metal contaminants; (3) scrubbing and neutralization of acidic vapors in flue gases or process-off gases; and (4) precipitation of anions like fluoride, phosphates and arsenic.
  • Magnesium hydroxide may be derived from three basic sources: seawater, well brines, and magnesia-bearing ores.
  • a magnesium hydroxide slurry is produced from the chemical reaction of dolime (also known as dolimitic quicklime, CaO MgO) and well brine according to the following: Reaction 1 : CaO MgO + 2H 2 0 ⁇ Mg(OH) 2 + Ca(OH) 2 Reaction 2: Mg(OH) 2 + Ca(OH) 2 + MgCI 2 - 2Mg(OH) 2 + CaCI 2
  • the well brine consists primarily of calcium chloride but also includes magnesium chloride.
  • the chemical reaction of dolime and well brine produces a slurry of magnesium hydroxide in a chloride-containing liquor.
  • the slurry is then further processed to reduce the chloride level and concentrated to approximately 40 to 65% magnesium hydroxide solids.
  • the magnesium hydroxide slurries prepared by these methods contain a small amount of Ca +2 , CaO and Ca(OH) 2 , each of which can be calculated according to the following:
  • a calcium oxide-based compound such as dolime or high calcium quicklime is added at some point during the manufacture of magnesium hydroxide slurry to form a high calcium hydroxide-containing Mg(OH) 2 slurry, i.e., a magnesium hydroxide slurry containing at least about 5.0% Ca(OH) 2 by weight.
  • a calcium-oxide based compound is hydrated with calcined magnesite, calcined brucite or MgO derived from other sources to form a slurry containing calcium hydroxide (Ca(OH) 2 ) and magnesium hydroxide (Mg(OH) 2 ).
  • the hydration can be carried out at increased pressure, for example, about 3 psig to about 150 psig, and preferably about 5 psig to about 70 psig, to increase the rate of hydration, or can be carried out at atmospheric pressure.
  • a calcium-oxide based compound is added to a previously manufactured magnesium hydroxide slurry product.
  • the calcium oxide-containing compound is not used to synthesize Mg(OH) 2 , but rather is used to supplement its reactivity in wastewater neutralization processes.
  • the hydration of calcined magnesite, calcined brucite or MgO can be achieved without dolime addition.
  • 5,514,357 discloses a method for producing a stabilized magnesium hydroxide slurry produced by conventional methods such as from well brine consisting of physically deflocculating the magnesium hydroxide solids in a starting slurry and optionally adding a cationic polymer and a thickening agent.
  • Witkowski et al. U.S. Patent No. 5,487,879, discloses a process for producing a moderate quality, pumpable, stabilized slurry of magnesium hydroxide from burnt natural magnesite that involves pressure hydrating a mixture containing burnt natural magnesite and water in the presence of chloride ions and cationic polymer.
  • magnesium hydroxide slurries prepared by the methods discussed above provide important advantages, such slurries, which are typically 95-98% Mg(OH) 2 by weight and 0.7 to 4.0% CaO by weight, are slow to react in wastewater when metal concentrations exceed several thousand parts per million as compared to the reaction rates obtained with lime or caustic soda due to their low solubility and buffering ability in water.
  • ash refuse leachate typically contains a high iron concentration that often exceeds 1000 ppm Fe +3 .
  • the pH increases and ferric iron precipitates as a hydroxide.
  • a "plateau effect" is observed in the neutralization curve as the pH remains constant until most of the ferric iron is removed.
  • the modified magnesium hydroxide slurry of the invention is particularly useful for the removal of high concentrations of metals from wastewater.
  • the modified magnesium hydroxide slurry of the invention is specifically useful in removing high concentrations of metals such as Fe +3 at wastewater concentrations greater than 250 ppm.
  • NaOH is readily soluble in water and is thus available for immediate reaction with acids, metals and anions.
  • Ca(OH) 2 is 20 times more soluble than Mg(OH) 2 and therefore reacts much faster.
  • magnesium hydroxide is consumed by acids, metals and anions, it must be replaced with fresh hydroxide, which must go from a solid form into a soluble form for reaction.
  • the rate of solubilization which is the rate controlling step, is controlled by the surface area of the magnesium hydroxide.
  • the sodium salts are very soluble so that NaOH does not precipitate anions and therefore does not permit their removal by filtration or sedimentation.
  • the calcium salts are slightly more insoluble than the magnesium salts.
  • a two-stage treatment system is utilized in some treatment systems where the sludge reduction benefits of standard magnesium hydroxide slurry are desired but the retention time of the system is too short for magnesium hydroxide to completely react.
  • the bulk of the neutralization and precipitation of metals is accomplished by using standard magnesium hydroxide slurry in a first stage and by adding a second alkali such as lime or caustic soda to ensure that the waste stream reaches the target pH in a second stage.
  • the ratio of the Mg(OH) 2 :Ca(OH) 2 or the Mg(OH) 2 :NaOH utilized must be regulated (i.e., the % by weight of magnesium hydroxide slurry must be greater than the % by weight of lime or caustic soda) in order to achieve significant sludge volume reduction.
  • the % by weight of magnesium hydroxide slurry must be greater than the % by weight of lime or caustic soda
  • the present inventors have therefore developed a modified magnesium hydroxide slurry which incorporates a high calcium hydroxide content to treat wastewater containing organic and inorganic acids (e.g., acetic acid, hydrofluoric acid, phosphoric acid, H 2 S0 4 , HCI, HBr, H 2 S0 3 , HN0 3 and citric acid), high concentrations of transition and heavy metals (e.g.
  • organic and inorganic acids e.g., acetic acid, hydrofluoric acid, phosphoric acid, H 2 S0 4 , HCI, HBr, H 2 S0 3 , HN0 3 and citric acid
  • transition and heavy metals e.g.
  • the modified magnesium hydroxide slurry of the invention provides a faster reacting product that can achieve a higher target pH than existing magnesium hydroxide slurries and thus permits improved heavy metal (specifically heavy metals having minimum solubility limits at pHs >8.5) and anion removal without compromising the sludge reduction benefits and superior sludge characteristics produced by the use of standard magnesium hydroxide slurry. In most cases, this faster reactivity reduces the amount of alkali slurry required to reach the target pH for optimum metal removal.
  • the present inventors have discovered a modified magnesium hydroxide slurry that is effective in many applications for water and wastewater treatment, such as acid neutralization and transition/heavy metal and anion precipitation.
  • a method for preparing a modified magnesium hydroxide slurry containing high calcium hydroxide content involves a magnesium hydroxide slurry manufacturing process, wherein, at some point during the process, a calcium oxide-based compound is added to the slurry to yield a slurry containing at least about 5.0% to about 30%, and more preferably between about 10% and about 30% Ca(OH) 2 , by weight (Mg(OH) 2 basis).
  • the method for preparing the modified magnesium hydroxide slurry of the invention involves modifications to manufacturing processes.
  • the method includes hydrating, and preferably, but not necessarily, pressure hydrating a mixture containing calcined natural magnesite, calcined brucite or MgO derived from other sources, with between about 5.0% and about 30%, and more preferably between about 10% and about 30% Ca(OH) 2 , by weight on a Mg(OH) 2 basis [wherein 10% Ca(OH) 2 by weight (Mg(OH) 2 basis) is equivalent to (1) 11.5% high calcium quicklime by weight (MgO basis) and (2) 18.5% dolime by weight (MgO basis)] and water in the presence of, for example, chloride ions and polymer.
  • polyaluminum chloride at about 0.4% to about 0.6% is used as the source of chloride ion.
  • the hydrated slurry is also preferably screened through 40 mesh to about 80 mesh sieves.
  • the Mg(OH) 2 agglomerates are preferably deagglomerated and dispersed by any suitable means.
  • a calcium oxide- containing compound is added to magnesium hydroxide slurry made by other means, including slurry made from brine/seawater and lime/dolime reactions carried out at atmospheric pressures and temperatures.
  • the increased calcium hydroxide concentration does not effect stability and resuspension characteristics of the modified magnesium hydroxide slurry.
  • the modified slurry exhibits good stability and resuspension characteristics when compared to standard magnesium hydroxide slurries.
  • the modified magnesium hydroxide slurry yields a % pour of >80% by slurry weight after 7 days of undisturbed gravity settling.
  • a process for treating wastewater using a modified magnesium hydroxide slurry containing high calcium hydroxide content is provided.
  • the process for treating wastewater using a modified magnesium hydroxide slurry having at least about 5.0% to about 30% Ca(OH) 2 by weight (Mg(OH) 2 basis) is particularly effective in acid neutralization and transition/heavy metal and anion precipitation.
  • the process permits higher reactivity in a shorter period of time and requires less modified magnesium hydroxide slurry by weight on an equivalent percent solids basis than standard magnesium hydroxide slurries to reach the same target pH.
  • the modified magnesium hydroxide slurry can be utilized in current wastewater treatment systems that are designed around lime or caustic soda reactivity.
  • a modified magnesium hydroxide slurry having at least about 5.0% to about 30%, and more preferably between about 10% and about 30% Ca(OH) 2 , by weight (Mg(OH) 2 basis) is provided.
  • the modified magnesium hydroxide slurry combines the sludge reduction benefits and superior sludge characteristics produced by the use of standard magnesium hydroxide slurry with improved acid neutralization and metal and anion precipitation.
  • the increased calcium hydroxide concentration does not effect the stability and resuspension characteristics of the modified magnesium hydroxide slurry relative to standard magnesium hydroxide slurries.
  • the modified magnesium hydroxide slurry of the invention may be stabilized with stabilizers that are known in the art as capable of maintaining stability and resuspension characteristics of the slurry.
  • stabilizers that are known in the art as capable of maintaining stability and resuspension characteristics of the slurry.
  • polyquatemary amine or alkylamine epichlorohydrin polymer may be added to the modified magnesium hydroxide slurry at some point during the manufacturing process to yield a % pour of >80% by slurry weight after 7 days of undisturbed gravity settling.
  • the modified magnesium hydroxide slurry of the invention provides important advantages, including faster acid neutralization and faster and more effective metal and anion removal when compared to standard magnesium hydroxide slurry.
  • FIG. 1 is a process flow diagram showing one example of a process for treating wastewater according to the present invention.
  • FIG. 2 is a pH vs. time plot showing the comparative buffering capacity on a 200% excess the stoichiometric basis (with respect to a NaOH titration on a representative sample of acid solution) for (1 ) 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) MagneClearTM58 and (2) modified magnesium hydroxide slurry containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis) for a 1N solution of hydrochloric acid.
  • FIG. 3 is a pH vs. time plot showing the comparative neutralization rates on a 15% to 40% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1) 90% Ca(OH) 2 by weight (hydroxide basis) (hydrated lime slurry), (2) 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) (MagneClearTM58) and (3) modified magnesium hydroxide slurry containing 5% Ca(OH) 2 by weight (Mg(OH) 2 basis).
  • FIG. 4 is a pH vs. time plot showing the comparative neutralization rates on a stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1) modified magnesium hydroxide slurry containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis), (2) 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) (MagneClearTM58) and (3) 0.92% Ca(OH) 2 by weight(Mg(OH) 2 basis)(FloMag®H).
  • FIG. 5 is a pH vs. time plot showing the comparative neutralization rates on a 50% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1) modified magnesium hydroxide slurry containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis), (2) 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) (MagneClearTM58) and (3) 0.92% Ca(OH) 2 by weight (Mg(OH) 2 basis) (FloMag®H).
  • FIG. 6 is a pH vs. time plot showing the comparative neutralization rates on a 30% to 50% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1 ) modified magnesium hydroxide slurry containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis), (2) 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) (MagneClearTM58), (3) 0.92% Ca(OH) 2 by weight (Mg(OH) 2 basis) (FloMag®H), (4) 96.66% Ca(OH) 2 by weight (hydroxide basis) (hydrated lime slurry) and (5) 55.16% Ca(OH) 2 by weight (Mg(OH) 2 basis) (hydrated dolime slurry).
  • FIG. 7 is a pH vs. time plot showing the comparative neutralization rates on a 30% excess the stoichiometric basis (with respect to a NaOH titration on a representative of wastewater sample) for (1) 90% Ca(OH) 2 by weight (Mg(OH) 2 basis) (hydrated lime slurry), (2) 5% Ca(OH) 2 by weight (Mg(OH) 2 basis), (3) 16% Ca(OH) 2 by weight (Mg(OH) 2 basis), (4) 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) (MagneClea ⁇ -TM58), (5) 9.0% Ca(OH) 2 by weight (Mg(OH) 2 basis) and (6) 45% excess the stoichiometric basis for 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) (MagneClearTM58).
  • FIG. 7 demonstrates that less amount of modified magnesium hydroxide slurry is required to reach the same pH than standard magnesium hydroxide slurry.
  • the process of the present invention can be used to treat, for example, acid mine drainage, coal pile runoff, hydrofluoric acid, and metal-plating waste streams.
  • the process involves the manufacture of a stabilized, pressure-hydrated magnesium hydroxide slurry as described in U.S. Patent No. 5,487,879, the entire contents of which are incorporated herein by reference.
  • 5,487,879 is modified according to the invention by adding at least about 5.0% to about 30% calcium hydroxide by weight (Mg(OH) 2 basis) derived from, for example, dolime, high calcium quicklime, hydrated lime, hydrated dolime, normal hydrated dolomitic lime (Type N), highly hydrated dolomitic lime (Type S) and any related byproducts (e.g. kiln dust fines, electrostatic precipitator dust, etc.) for the purpose of enhancing reactivity when neutralizing wastewater.
  • Mg(OH) 2 basis calcium hydroxide by weight derived from, for example, dolime, high calcium quicklime, hydrated lime, hydrated dolime, normal hydrated dolomitic lime (Type N), highly hydrated dolomitic lime (Type S) and any related byproducts (e.g. kiln dust fines, electrostatic precipitator dust, etc.) for the purpose of enhancing reactivity when neutralizing wastewater.
  • the pressure used in the pressure-hydration reaction is determined by the material and the construction of the reactors used and not the process.
  • the rate of cooling can be adjusted to keep the pressure under a maximum.
  • the end product is a modified magnesium hydroxide slurry of "moderate quality" (i.e., about 55-65% solids content by total slurry weight, Brookfield viscosity at room temperature of 50-500 cps, pourability/flowability > 80% by weight of sample poured off after 7 days of undisturbed (unagitated) gravity settling, water separation ⁇ 1 inch (wherein, the height of water separation is measured in a standard cylindrical 8 oz. polyethylene bottle (2 in. OD x 5 3/8 in. height) and settled solids which are readily resuspendable). Resuspension characteristics were determined by the ability to resuspend any settled solids with a glass probe without difficulty.
  • moderate quality i.e., about 55-65% solids content by total slurry weight, Brookfield viscosity at room temperature of 50-500 cps, pourability/flowability > 80% by weight of sample poured off after 7 days of undisturbed (unagitated) gravity settling, water separation
  • the process involves the manufacture of a stabilized, hydrated magnesium hydroxide slurry that is hydrated at atmospheric pressures.
  • a third process for preparing a modified magnesium hydroxide slurry according to the invention is described below.
  • the process involves mixing a calcium oxide-based compound such as high calcium quicklime or dolime with diluted magnesium hydroxide slurry synthesized from any known method, e.g., the hydration methods discussed above and the brine/seawater and lime/dolime reactions disclosed in U.S. Patents Nos. 4,314,985, 5,514,357 and 3,301 ,633, the entire contents of which are incorporated by reference herein.
  • the process involves:
  • Additional reactivity of the modified magnesium hydroxide slurry of the invention can be achieved by adding sodium hydroxide to the modified magnesium hydroxide slurry of the invention.
  • sodium hydroxide solubilizes quickly in water.
  • adding a percentage of NaOH 1-10% by weight to magnesium hydroxide slurry would result in an increased rate of reactivity and a higher endpoint pH.
  • modified magnesium hydroxide slurry By using the modified magnesium hydroxide slurry according to the invention, wastewater, and in particular wastewater containing high metal concentrations, can be treated more economically, since the modified magnesium hydroxide slurry (1) reduces the amount of slurry required to reach a target pH when compared to standard magnesium hydroxide slurry and (2) reduces the amount of sludge generated for disposal during metal precipitation when compared to lime or caustic soda.
  • a typical wastewater treatment system 1 is shown.
  • transition or heavy metals (not shown) are removed via hydroxide precipitation.
  • the waste stream 2 is separated from the return sludge 9, which is discharged as waste sludge 10 for further processing or disposal, and neutralized with an alkali 3 such as magnesium hydroxide slurry, lime or caustic soda in a neutralization tank 4 for a specified amount of time (i.e., retention time).
  • the retention time of the system 1 is built into the design of the treatment system 1 and depends on the wastewater flow rate and the size of the neutralization tank 4.
  • a pH probe 5 is immersed in the neutralization tank 4 to monitor the rate of neutralization and indicate when more alkali 3 is needed to reach a target pH.
  • the target pH usually corresponds to the pH of the minimum solubility for a particular metal hydroxide in water.
  • manganese has a minimum solubility in water at a pH >8.5.
  • a polymer 6 e.g., flocculent
  • the waststream 2 may be flocculated in a flocculator 11 in order to agglomerate smaller metal hydroxide particles into larger particles for faster settling.
  • the settled metal hydroxide particles (not shown) are collected at the bottom of a clarifier 7 as waste sludge 10 for further processing or disposal.
  • the clarified filtrate (not shown) is then properly discharged as effluent 8.
  • Table 1 below shows the particle sizes of ferric hydroxide sludge generated after neutralization of an acidic solution containing iron using caustic soda, hydrated lime, standard magnesium hydroxide slurry, and the modified magnesium hydroxide slurry of the invention.
  • Table 1 exemplifies the sludge benefits of the modified magnesium hydroxide slurry over lime and caustic soda.
  • the standard and the modified magnesium hydroxide slurries produce sludge crystals having larger particles than lime and caustic soda. The resulting sludge is more dense and compact and thus, results in lower sludge volume.
  • the larger sludge particle produced by standard Mg(OH) 2 slurry and the modified Mg(OH) 2 slurry eliminates the need to recycle sludge as frequently done in lime systems to build larger sludge particles for better clarification and filtration. Additional equipment for recycling sludge is therefore eliminated which results in reduced capital costs.
  • magnesium hydroxide forms metallic hydroxide sludge, which has a higher density, a higher solids content and a smaller volume than sludge produced from lime or caustic soda due to a larger metallic hydroxide particle size as compared to smaller metallic hydroxides made from lime or caustic soda.
  • Lime and caustic soda cause flash
  • the modified magnesium hydroxide slurry of the invention to treat wastewater is advantageous over the use of lime and caustic soda.
  • the modified magnesium hydroxide slurry of the invention buffers to a lower pH and thus provides better pH control. Adding up to about 30% Ca(OH) 2 by weight
  • magnesium hydroxide slurry into magnesium hydroxide slurry does not adversely affect sludge volume or density, while significantly reducing the reaction time when compared to standard magnesium hydroxide slurry.
  • use of the modified magnesium hydroxide slurry of the invention is also advantageous over use of standard magnesium hydroxide slurry.
  • the modified magnesium hydroxide slurry of the invention enhances reactivity in wastewater neutralization systems, particularly in the presence of high amounts of transition and heavy metals such as Fe +3 , Mn +2 , Cu +2 , Cr +3 , Ni +2 , etc.
  • transition and heavy metals such as Fe +3 , Mn +2 , Cu +2 , Cr +3 , Ni +2 , etc.
  • the plants generally have limited retention time in neutralization tanks and thus frequently permit too short a reaction time for standard magnesium hydroxide slurries to be effective.
  • the alkalinity of the treated water often is too low for manganese and other metals to precipitate within the allotted reaction time.
  • Adding small quantities of calcium hydroxide to the magnesium hydroxide slurry causes the slurry to react faster without jeopardizing the advantage of more compact sludge compared to the gelatinous and voluminous sludge produced by lime or caustic soda.
  • modified magnesium hydroxide slurry in accordance with the invention in treating wastewater containing high amounts of transition and heavy metals is the ability to achieve a higher endpoint pH, while requiring less amount of slurry on a weight % basis than standard magnesium hydroxide slurry.
  • improved metal removal can occur, particularly with those metals having minimum solubilities at pHs >8.5.
  • modified magnesium hydroxide slurry include:
  • a laboratory autoclave was charged with calcined natural magnesite, high calcium quicklime or dolime, water, and polyaluminum chloride.
  • Various ratios of calcined natural magnesite to high calcium quicklime or dolime were used to provide a minimum of about 5.0% Ca(OH) 2 by weight (Mg(OH) 2 basis) and a maximum of about 30% Ca(OH) 2 by weight (Mg(OH) 2 basis).
  • Approximately 0.4 to 0.6% by weight (slurry basis) of polyaluminum chloride was added to increase the degree of hydration of calcined magnesite and reduce the viscosity of the resulting slurry.
  • a suitable polymer for use with this invention either alkylamine epichlorohydrin polymer or polyquatemary amine was added in an amount equal to 0.02 - 0.06% by weight (slurry basis) to the pressure hydrated slurry while blending on low speed with a Shar dispersion mixer.
  • the slurry was then screened through 40 mesh (i.e., 420 microns) and 80 mesh (i.e., 177 microns) sieves and processed through an APV Gaulin 15MR-8TA laboratory homogenizer at pressures ranging from about 1000 to 2500 psig and at two or three passes to provide better particle dispersion and a more flowable product.
  • Stability tests on the modified magnesium hydroxide slurry showed encouraging pour results, i.e., >80% by weight of slurry poured after 7 days of undisturbed gravity settling and ⁇ 1 inch water separation, and sufficient stability for transportation by truckloads.
  • modified magnesium hydroxide slurry formulations were developed, stabilized, and tested.
  • Table 2 shows modified slurry formulations and their degree of stability as measured in % pour, which was determined according to the following: weight of slurry poured off from an 8 oz. bottle after 30 seconds X 100 total weight of slurry
  • FIG. 2 shows comparative buffering capacities between MagneClearTM58 Magnesium Hydroxide Slurry and the modified magnesium hydroxide slurry prepared according to Example 1 of the invention containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis) for a 1N solution of hydrochloric acid.
  • FIG. 3 illustrates the rate of neutralization for 90% Ca(OH) 2 by weight (hydroxide basis) hydrated lime slurry, 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) MagneClearTM58 Magnesium Hydroxide Slurry and 5% Ca(OH) 2 by weight (Mg(OH) 2 basis) modified magnesium hydroxide slurry.
  • the untreated wastewater sample was raised to a pH of 8.3 with hydrated lime slurry and to a pH of 8.0 with MagneClearTM58.
  • This graph shows that the modified magnesium hydroxide slurry, which incorporates a percentage of dolime, (i.e., 10% dolime by weight (MgO basis)) has an increased rate of neutralization when compared to MagneClearTM58.
  • FIGS. 4 and 5 respectively, compare neutralization rates on a stoichiometric basis and a 50% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) between modified magnesium hydroxide slurry containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis) and two standard magnesium hydroxide slurries, FloMag®H, a synthetic slurry produced from brine and dolime having a typical Ca(OH) 2 content of about 0.92% by weight (Mg(OH) 2 basis), and MagneClearTM58, a slurry produced from pressure hydrating naturally occurring calcined magnesite and water having a typical Ca(OH) 2 content of about 1.98% by weight (Mg(OH) 2 basis).
  • modified magnesium hydroxide slurry containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis) and two standard magnesium hydroxide slurries, FloMag®H, a synthetic
  • the wastewater in this case is ash refuse leachate containing approximately 1200 ppm ferric iron and 48 ppm manganese.
  • the graphs in FIGS. 4 and 5 show that the modified magnesium hydroxide slurry of the invention neutralizes the leachate containing a high iron concentration faster than and achieves an endpoint pH after 150 minutes of retention time higher than both of the standard magnesium hydroxide slurries.
  • Table 4 shows (1) ferric iron concentration, (2) manganese concentration and (3) endpoint pH after treatment with each alkali.
  • the modified magnesium hydroxide slurry improved manganese removal by reducing Mn +2 from 48 ppm to 1.1 ppm due to the higher endpoint pH.
  • the standard magnesium hydroxide slurries were ineffective in lowering manganese to low concentrations as shown by the 7 ppm and 9.2 ppm manganese concentrations for FloMag®H and MagneClearTM58, respectively.
  • Neutralization tests were run on an ash refuse leachate wastewater sample containing high iron and manganese concentrations (i.e., 1200 ppm Fe +3 and 48 ppm Mn +2 ).
  • the following alkalis were utilized for neutralization and metal precipitation: (1) a modified magnesium hydroxide slurry containing 10.6% Ca(OH) 2 by weight (Mg(OH) 2 basis), (2) 0.92% Ca(OH) 2 by weight (Mg(OH) 2 basis) (FloMag®H), (3) 1.98% Ca(OH) 2 by weight (Mg(OH) 2 basis) (MagneClearTM58), (4) 96.66% Ca(OH) 2 by weight (hydroxide basis) (hydrated lime slurry) and (5) 55.16% Ca(OH) 2 by weight (Mg(OH) 2 basis) (hydrated dolime slurry).
  • the modified magnesium hydroxide slurry increased the rate of neutralization and reached a higher endpoint pH of 8.48 versus 7.9 for FloMag®H and 8.08 for MagneClearTM58 Magnesium Hydroxide Slurries.
  • less modified magnesium hydroxide slurry was required (by weight) than standard magnesium hydroxide slurry to reach the same target pH, and improved manganese removal was achieved due to the higher reactivity (see Table 5).
  • the modified magnesium hydroxide slurry reduced the amount of sludge volume generated by 78% and 44% when compared to treatment with hydrated lime slurry and hydrated dolime slurry, respectively.
  • Table 5 shows (1) ferric ion concentration, (2) manganese concentration, (3) sludge volume and (4) endpoint pH after treatment with each alkali.
  • Neutralization tests on a fresh sample of coal pile leachate were conducted to further compare slurry reactivity.
  • Alkali was added immediately to the untreated coal pile leachate sample. After 80 minutes of rapid agitation on a stir plate, air was bubbled through the beaker to simulate aeration in the second stage reactor (neutralization tank). In this treatment system, alkali is added in the first neutralization tank while aeration is added in the second neutralization tank. This aeration along with rapid agitation were applied for an additional 138 minutes for a total neutralization time of 218 minutes.
  • FIG. 7 illustrates the rate of neutralization for this particular wastewater sample.
  • modified magnesium hydroxide slurry formulations which incorporate dolime or high calcium quicklime. Fifteen percent less, i.e., 30% excess (on slurry weight basis) of the modified magnesium hydroxide slurry blends is required to reach the same pH as a 45% excess the stoichiometric amount of MagneClearTM58.
  • Slurries are expressed as % Ca(OH) 2 by wt. on a hydroxide basis.
  • Dolime and High Calcium Quicklime (i.e., Hi-Cal Lime) percentages are on a MgO basis.
  • MagneClearTM58 and each of the modified magnesium hydroxide slurry formulations achieve good iron removal resulting in less than 1.0 ppm Fe +3 content.
  • the modified magnesium hydroxide slurries of the invention exhibited improved manganese removal.
  • the 5% Ca(OH) 2 by weight (Mg(OH) 2 basis) modified magnesium hydroxide slurry reduced the manganese from 44 ppm to 1.9 ppm; the 9% Ca(OH) 2 by weight (Mg(OH) 2 basis) modified magnesium hydroxide slurry proved to be even more effective in reducing manganese to 1.4 ppm; and the 16% Ca(OH) 2 by weight (Mg(OH) 2 basis) modified magnesium hydroxide slurry achieved 0.04 ppm manganese.
  • Example 5 a significant sludge volume reduction is achieved using MagneClearTM58 and the modified magnesium hydroxide slurry formulations according to the invention to neutralize coal pile leachate.
  • Table 7 shows the % reduction in sludge volume for each slurry when compared to hydrated lime slurry:
  • modified magnesium hydroxide slurries of the invention have (1) increased rates of neutralization, (2) improved manganese removal, (3) reduction in alkali requirement on a slurry weight basis when compared to MagneClearTM58 Magnesium Hydroxide Slurry, and (4) significant sludge volume reduction when compared to hydrated lime slurry.
  • the embodiments described above provide a number of significant advantages, including the use of a modified magnesium hydroxide slurry that allows a more rapid and effective treatment of wastewater to be conducted using current wastewater treatment systems.

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Abstract

A process for preparing a modified magnesium hydroxide slurry to be used for treating waste water and a method for treating wastewater using a modified magnesium hydroxide slurry containing a high calcium hydroxide content are described. In a typical wastewater treatment system (1), transition or heavy metals are removed via hydroxide precipitation. The waste stream (2) is separated from the return sludge (9), which is discharged as waste sludge (10) for further processing or disposal, and neutralized with an alkali (3) in a neutralization tank (4), in which is immersed pH probe (5). A polymer (6) may be added to the waste stream (2) and the waste stream (2) may be flocculated in a flocculator (11) in order to agglomerate smaller metal hydroxide particles into larger particles for faster settling. The settled metal hydroxide particles are collected at the bottom of a clarifier (7) as waste sludge (10) for further processing or disposal. The clarified filtrate is then properly discharged as effluent (8).

Description

A MODIFIED MAGNESIUM HYDROXIDE SLURRY FOR USE IN TREATING WASTEWATER AND A PROCESS FOR PRODUCING THEREOF
Field of the Invention
The invention relates to processes for preparing a modified magnesium hydroxide slurry to be used for treating wastewater, and more particularly to processes for treating wastewater using a modified magnesium hydroxide slurry. The invention encompasses the use of a stable modified magnesium hydroxide slurry containing high calcium hydroxide content that achieves 1 ) effective metal removal to meet discharge limits, 2) significant sludge volume reduction when compared to the amount of sludge generated by lime and caustic soda, particularly in sulfate systems, and 3) increased rate of neutralization compared to standard magnesium hydroxide slurry. Use of the modified magnesium hydroxide slurry according to the invention to treat wastewater is advantageous over the use of, for example, lime, caustic soda and standard magnesium hydroxide slurry. The modified magnesium hydroxide slurry of the invention is advantageous over lime and caustic soda in that it produces less sludge volume and sludge having a higher density and solids content, which makes filtration easier. In addition, unlike lime and caustic soda, the modified magnesium hydroxide slurry of the invention buffers to a lower pH. The modified magnesium hydroxide slurry of the invention is advantageous over standard magnesium hydroxide slurry in that it achieves a higher endpoint pH, provides more efficient metal removal, requires less slurry by weight and significantly reduces the reaction time.
Background of the Invention
Magnesium hydroxide in slurry form is useful as a pumpable source of magnesium hydroxide for various chemical processes, including but not limited to the following: (1) pH adjustment, including waste acid and acidic wastewater neutralization; (2) wastewater treatment, including precipitation of heavy metal contaminants; (3) scrubbing and neutralization of acidic vapors in flue gases or process-off gases; and (4) precipitation of anions like fluoride, phosphates and arsenic. Magnesium hydroxide may be derived from three basic sources: seawater, well brines, and magnesia-bearing ores. In a typical synthetic process, a magnesium hydroxide slurry is produced from the chemical reaction of dolime (also known as dolimitic quicklime, CaO MgO) and well brine according to the following: Reaction 1 : CaO MgO + 2H20 → Mg(OH)2 + Ca(OH)2 Reaction 2: Mg(OH)2 + Ca(OH)2 + MgCI2 - 2Mg(OH)2 + CaCI2 The well brine consists primarily of calcium chloride but also includes magnesium chloride. The chemical reaction of dolime and well brine produces a slurry of magnesium hydroxide in a chloride-containing liquor. The slurry is then further processed to reduce the chloride level and concentrated to approximately 40 to 65% magnesium hydroxide solids. The magnesium hydroxide slurries prepared by these methods contain a small amount of Ca+2, CaO and Ca(OH)2, each of which can be calculated according to the following:
1. Determine the amount of Ca+2 in a dried sample of Mg(OH)2 by the inductively coupled plasma unit.
2. Determine the amount of CaO according to the following: Result of % Ca+2 X 56.079 g/mol CaO = % CaO (by weight).
40.80 g/mol Ca
3. Determine the amount of Ca(OH)2 according to the following: % CaO (by weight) X 74.94 g/mol Ca(OH)2 = % Ca(OH)2 (by weight)
56.079 g/mol CaO
According to the present invention, a calcium oxide-based compound such as dolime or high calcium quicklime is added at some point during the manufacture of magnesium hydroxide slurry to form a high calcium hydroxide-containing Mg(OH)2 slurry, i.e., a magnesium hydroxide slurry containing at least about 5.0% Ca(OH)2 by weight.
According to one embodiment, a calcium-oxide based compound is hydrated with calcined magnesite, calcined brucite or MgO derived from other sources to form a slurry containing calcium hydroxide (Ca(OH)2) and magnesium hydroxide (Mg(OH)2). The hydration can be carried out at increased pressure, for example, about 3 psig to about 150 psig, and preferably about 5 psig to about 70 psig, to increase the rate of hydration, or can be carried out at atmospheric pressure.
According to another embodiment, a calcium-oxide based compound is added to a previously manufactured magnesium hydroxide slurry product. In both cases, the calcium oxide-containing compound is not used to synthesize Mg(OH)2, but rather is used to supplement its reactivity in wastewater neutralization processes. The hydration of calcined magnesite, calcined brucite or MgO can be achieved without dolime addition.
Bonney, U.S. Patent No. 4,314,985, discloses a process for recovering magnesium hydroxide from sea water. Miyata et al., U.S. Patent No. 4,472,370, discloses a process for producing magnesium hydroxide from calcium oxide and magnesium chloride or magnesium nitrate in an aqueous medium. Stowe et al., U.S. Patent No. 3,301 ,633, discloses the production of magnesium hydroxide and calcium chloride from a brine containing magnesium chloride and a form of quicklime selected from the group consisting of calcined limestone and dolomite. Richmond et al., U.S. Patent No. 5,514,357, discloses a method for producing a stabilized magnesium hydroxide slurry produced by conventional methods such as from well brine consisting of physically deflocculating the magnesium hydroxide solids in a starting slurry and optionally adding a cationic polymer and a thickening agent. Witkowski et al., U.S. Patent No. 5,487,879, discloses a process for producing a moderate quality, pumpable, stabilized slurry of magnesium hydroxide from burnt natural magnesite that involves pressure hydrating a mixture containing burnt natural magnesite and water in the presence of chloride ions and cationic polymer.
While the magnesium hydroxide slurries prepared by the methods discussed above provide important advantages, such slurries, which are typically 95-98% Mg(OH)2 by weight and 0.7 to 4.0% CaO by weight, are slow to react in wastewater when metal concentrations exceed several thousand parts per million as compared to the reaction rates obtained with lime or caustic soda due to their low solubility and buffering ability in water. For example, ash refuse leachate typically contains a high iron concentration that often exceeds 1000 ppm Fe+3. When this waste stream is treated with standard magnesium hydroxide slurry, the pH increases and ferric iron precipitates as a hydroxide. A "plateau effect" is observed in the neutralization curve as the pH remains constant until most of the ferric iron is removed.
When treating such a high concentration of metallic ion, standard magnesium hydroxide slurries form metallic hydroxide sludge, which has a higher density, a higher solids content and a smaller volume than sludge produced from lime or caustic soda due to a larger metallic hydroxide particle size as compared to smaller metallic hydroxides made from lime or caustic soda. Therefore, neutralization and precipitation using such Mg(OH)2 slurries requires a sufficiently greater reaction time, which varies with the wastewater system employed, than for neutralization and precipitation of these metals using lime or caustic soda. However, lime and caustic soda cause immediate precipitation of the metallic hydroxides, which although permits rapid neutralization and precipitation, results in extremely fine particles that are gelatinous and difficult to separate either by filtration or settling. The addition of up to about 30% Ca(OH)2 by weight on a Mg(OH)2 basis, in contrast, does not adversely affect sludge volume or density, while significantly reducing the amount of slurry by weight required, if comparable weights are used, significantly reducing the reaction time when compared to standard magnesium hydroxide slurry. As a result, the reduction in sludge volume reduces the overall costs associated with sludge disposal.
The modified magnesium hydroxide slurry of the invention is particularly useful for the removal of high concentrations of metals from wastewater. The modified magnesium hydroxide slurry of the invention is specifically useful in removing high concentrations of metals such as Fe+3 at wastewater concentrations greater than 250 ppm.
The majority of existing wastewater treatment systems are designed around lime or caustic soda reactivity. As disclosed in an article entitled "Pollution Rx" 'Milk of Magnesia'" by John Teringo, Products Finishing, August 1987, since lime (e.g., Ca(OH)2) and caustic soda (NaOH) are more soluble than magnesium hydroxide, they dissociate and provide hydroxyl ions more rapidly when added to wastewater. The following summarizes the solubility of (1 ) hydrated lime (Ca(OH)2), NaOH and Mg(OH)2 in grams per 100 grams of water:
Temperature, °C Ca(OH)2 NaOH Mg(OH)2
0 0.185 42
18 - 0.0009
100 0.077 347 0.0040
As can be seen from the data above, NaOH is readily soluble in water and is thus available for immediate reaction with acids, metals and anions. Ca(OH)2 is 20 times more soluble than Mg(OH)2 and therefore reacts much faster. As magnesium hydroxide is consumed by acids, metals and anions, it must be replaced with fresh hydroxide, which must go from a solid form into a soluble form for reaction. The rate of solubilization, which is the rate controlling step, is controlled by the surface area of the magnesium hydroxide.
Since lime and NaOH are more soluble than magnesium hydroxide, hydroxyl ions solubilize at a faster rate than Mg(OH)2, which, in turn, allows little or no time for crystal growth or agglomeration during precipitation of metal ions. The result is a precipitate of very small particles. The particles, which are combined in a web-like structure that traps water and forms a gel-like sludge, are hard to filter. Thus, a large amount of water remains in the sludge after filtration. In contrast, magnesium hydroxide, which is much less soluble, slows the development of metal hydroxides enough to allow crystal growth. The large crystals created when using magnesium hydroxide produce a more dense sludge that is easier to dewater. Due to the high density and low water content, the volume of dewatered sludge produced is considerably less than that produced by an equivalent system using lime or NaOH.
Metals and acids react with the hydroxide portion of lime, NaOH and magnesium hydroxide to form insoluble metallic hydroxides or soluble neutral salts. Anions react with the hydroxide portion of lime, NaOH and magnesium hydroxide to form insoluble magnesium salts. The following summarizes the solubility of various anions in grams per 100 grams of water containing (1) Ca(OH)2) (2) NaOH and (3) Mg(OH)2: fluoride triphosphate arsenate
Ca(OH)2 0.001618 0.00225 0.01325
NaOH 1 225 8.8 38.915
Mg(OH)2 0.007618 0.0205 insoluble
The sodium salts are very soluble so that NaOH does not precipitate anions and therefore does not permit their removal by filtration or sedimentation. The calcium salts are slightly more insoluble than the magnesium salts. By the addition of lime to magnesium hydroxide slurry according to the present invention, a modified magnesium hydroxide slurry that is more efficient at removing acids, metals and anions than standard magnesium hydroxide slurry is available.
Current wastewater treatment systems that are designed around lime and caustic soda frequently do not provide adequate retention time for Mg(OH)2 to completely react and provide sufficient metal removal. The influent and effluent flow rates in such wastewater treatment systems, which depend upon factors such as the processes upstream, the plant's capacity, the amount of rainfall during a certain period, the degree of water reuse and the type of contaminants in the wastewater, vary from plant to plant. As disclosed in Metcalf and Eddy, Wastewater Engineering, McGraw-Hill, Inc., 1991 , typical design values for estimating the flow rates from industrial areas that have little or no wet process type industries are 1000 - 1500 gallons/acre-day for light industrial developments and 1500 - 3000 gallons/acre-day for medium industrial developments. It is also known that some industries treat in excess of 1 million gallons/acre-day. Thus, such systems do not provide efficient and cost-effective use of the currently available magnesium hydroxide slurries. A two-stage treatment system is utilized in some treatment systems where the sludge reduction benefits of standard magnesium hydroxide slurry are desired but the retention time of the system is too short for magnesium hydroxide to completely react. The bulk of the neutralization and precipitation of metals is accomplished by using standard magnesium hydroxide slurry in a first stage and by adding a second alkali such as lime or caustic soda to ensure that the waste stream reaches the target pH in a second stage. In such two-stage systems, the ratio of the Mg(OH)2:Ca(OH)2 or the Mg(OH)2:NaOH utilized must be regulated (i.e., the % by weight of magnesium hydroxide slurry must be greater than the % by weight of lime or caustic soda) in order to achieve significant sludge volume reduction. However, since many existing wastewater treatment systems are designed around a single chemical feed system, additional capital would be required to modify such systems to achieve a two-stage system. According to the present invention, however, since only one chemical need be used, only one feed system is needed, thus, reducing the overall treatment costs.
The present inventors have therefore developed a modified magnesium hydroxide slurry which incorporates a high calcium hydroxide content to treat wastewater containing organic and inorganic acids (e.g., acetic acid, hydrofluoric acid, phosphoric acid, H2S04, HCI, HBr, H2S03, HN03 and citric acid), high concentrations of transition and heavy metals (e.g. Fe+3, Fe+2, Ni+2, Ag+2, Mn+2, Co, Ail, Ti, Mo, Pt, Cd+2, Al+3, Cu+2, Zn+2, Pb+2, Cr+3, Sb, Hg, Sn, and Bi), and/or anions such as fluorides, phosphates and arsenic. The modified magnesium hydroxide slurry of the invention provides a faster reacting product that can achieve a higher target pH than existing magnesium hydroxide slurries and thus permits improved heavy metal (specifically heavy metals having minimum solubility limits at pHs >8.5) and anion removal without compromising the sludge reduction benefits and superior sludge characteristics produced by the use of standard magnesium hydroxide slurry. In most cases, this faster reactivity reduces the amount of alkali slurry required to reach the target pH for optimum metal removal.
None of the references described above provides the important advantages of using a modified magnesium hydroxide slurry that has a high calcium hydroxide content to more rapidly and, thus, more effectively, remove high concentrations of acids, metals and anions from wastewater. Also, none of the patents described above provides a modified magnesium hydroxide slurry that incorporates a high calcium hydroxide content to treat wastewater without compromising the sludge reduction benefits and superior sludge characteristics produced by using standard magnesium hydroxide slurry.
Summary of the Invention
The present inventors have discovered a modified magnesium hydroxide slurry that is effective in many applications for water and wastewater treatment, such as acid neutralization and transition/heavy metal and anion precipitation.
According to one embodiment of the invention, a method for preparing a modified magnesium hydroxide slurry containing high calcium hydroxide content is provided. The method involves a magnesium hydroxide slurry manufacturing process, wherein, at some point during the process, a calcium oxide-based compound is added to the slurry to yield a slurry containing at least about 5.0% to about 30%, and more preferably between about 10% and about 30% Ca(OH)2, by weight (Mg(OH)2 basis).
As pointed out above, the method for preparing the modified magnesium hydroxide slurry of the invention involves modifications to manufacturing processes. According to one embodiment of the invention, the method includes hydrating, and preferably, but not necessarily, pressure hydrating a mixture containing calcined natural magnesite, calcined brucite or MgO derived from other sources, with between about 5.0% and about 30%, and more preferably between about 10% and about 30% Ca(OH)2, by weight on a Mg(OH)2 basis [wherein 10% Ca(OH)2 by weight (Mg(OH)2 basis) is equivalent to (1) 11.5% high calcium quicklime by weight (MgO basis) and (2) 18.5% dolime by weight (MgO basis)] and water in the presence of, for example, chloride ions and polymer. Preferably polyaluminum chloride at about 0.4% to about 0.6% is used as the source of chloride ion. Preferably about 0.02% to about 0.06% by weight (slurry basis), and more preferably about 0.02% by weight (slurry basis), polyquatemary amine or alkylamine epichlorohydrin, is used as the polymer.
The hydrated slurry is also preferably screened through 40 mesh to about 80 mesh sieves. In addition, the Mg(OH)2 agglomerates are preferably deagglomerated and dispersed by any suitable means. According to another embodiment of the invention, a calcium oxide- containing compound is added to magnesium hydroxide slurry made by other means, including slurry made from brine/seawater and lime/dolime reactions carried out at atmospheric pressures and temperatures.
As discussed above, the increased calcium hydroxide concentration does not effect stability and resuspension characteristics of the modified magnesium hydroxide slurry. In fact, the modified slurry exhibits good stability and resuspension characteristics when compared to standard magnesium hydroxide slurries. When stabilized with about 0.02% to about 0.06% by weight (slurry basis) polyquatemary amine, the modified magnesium hydroxide slurry yields a % pour of >80% by slurry weight after 7 days of undisturbed gravity settling.
Also according to the invention, a process for treating wastewater using a modified magnesium hydroxide slurry containing high calcium hydroxide content is provided. As discussed above, the process for treating wastewater using a modified magnesium hydroxide slurry having at least about 5.0% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis) is particularly effective in acid neutralization and transition/heavy metal and anion precipitation. For example, the process permits higher reactivity in a shorter period of time and requires less modified magnesium hydroxide slurry by weight on an equivalent percent solids basis than standard magnesium hydroxide slurries to reach the same target pH.
In addition, by having a quicker reaction time, the modified magnesium hydroxide slurry can be utilized in current wastewater treatment systems that are designed around lime or caustic soda reactivity.
Also, according to the invention, a modified magnesium hydroxide slurry having at least about 5.0% to about 30%, and more preferably between about 10% and about 30% Ca(OH)2 , by weight (Mg(OH)2 basis) is provided. The modified magnesium hydroxide slurry combines the sludge reduction benefits and superior sludge characteristics produced by the use of standard magnesium hydroxide slurry with improved acid neutralization and metal and anion precipitation. In addition, to a point, the increased calcium hydroxide concentration does not effect the stability and resuspension characteristics of the modified magnesium hydroxide slurry relative to standard magnesium hydroxide slurries. The modified magnesium hydroxide slurry of the invention may be stabilized with stabilizers that are known in the art as capable of maintaining stability and resuspension characteristics of the slurry. For example, about 0.02% to about 0.06% by weight (slurry basis) polyquatemary amine or alkylamine epichlorohydrin polymer may be added to the modified magnesium hydroxide slurry at some point during the manufacturing process to yield a % pour of >80% by slurry weight after 7 days of undisturbed gravity settling.
As pointed out in greater detail below, the modified magnesium hydroxide slurry of the invention provides important advantages, including faster acid neutralization and faster and more effective metal and anion removal when compared to standard magnesium hydroxide slurry.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed descriptions, taken in conjunction with the accompanying drawings.
Brief Description of the Drawings
FIG. 1 is a process flow diagram showing one example of a process for treating wastewater according to the present invention.
FIG. 2 is a pH vs. time plot showing the comparative buffering capacity on a 200% excess the stoichiometric basis (with respect to a NaOH titration on a representative sample of acid solution) for (1 ) 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) MagneClear™58 and (2) modified magnesium hydroxide slurry containing 10.6% Ca(OH)2by weight (Mg(OH)2 basis) for a 1N solution of hydrochloric acid.
FIG. 3 is a pH vs. time plot showing the comparative neutralization rates on a 15% to 40% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1) 90% Ca(OH)2 by weight (hydroxide basis) (hydrated lime slurry), (2) 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) (MagneClear™58) and (3) modified magnesium hydroxide slurry containing 5% Ca(OH)2 by weight (Mg(OH)2 basis).
FIG. 4 is a pH vs. time plot showing the comparative neutralization rates on a stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1) modified magnesium hydroxide slurry containing 10.6% Ca(OH)2 by weight (Mg(OH)2 basis), (2) 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) (MagneClear™58) and (3) 0.92% Ca(OH)2 by weight(Mg(OH)2 basis)(FloMag®H).
FIG. 5 is a pH vs. time plot showing the comparative neutralization rates on a 50% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1) modified magnesium hydroxide slurry containing 10.6% Ca(OH)2 by weight (Mg(OH)2 basis), (2) 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) (MagneClear™58) and (3) 0.92% Ca(OH)2 by weight (Mg(OH)2 basis) (FloMag®H).
FIG. 6 is a pH vs. time plot showing the comparative neutralization rates on a 30% to 50% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) for (1 ) modified magnesium hydroxide slurry containing 10.6% Ca(OH)2 by weight (Mg(OH)2 basis), (2) 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) (MagneClear™58), (3) 0.92% Ca(OH)2 by weight (Mg(OH)2 basis) (FloMag®H), (4) 96.66% Ca(OH)2 by weight (hydroxide basis) (hydrated lime slurry) and (5) 55.16% Ca(OH)2 by weight (Mg(OH)2 basis) (hydrated dolime slurry).
FIG. 7 is a pH vs. time plot showing the comparative neutralization rates on a 30% excess the stoichiometric basis (with respect to a NaOH titration on a representative of wastewater sample) for (1) 90% Ca(OH)2 by weight (Mg(OH)2 basis) (hydrated lime slurry), (2) 5% Ca(OH)2 by weight (Mg(OH)2 basis), (3) 16% Ca(OH)2 by weight (Mg(OH)2 basis), (4) 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) (MagneCleaι-™58), (5) 9.0% Ca(OH)2 by weight (Mg(OH)2 basis) and (6) 45% excess the stoichiometric basis for 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) (MagneClear™58). FIG. 7 demonstrates that less amount of modified magnesium hydroxide slurry is required to reach the same pH than standard magnesium hydroxide slurry. Detailed Description of the Invention
The process of the present invention can be used to treat, for example, acid mine drainage, coal pile runoff, hydrofluoric acid, and metal-plating waste streams. According to one embodiment, the process involves the manufacture of a stabilized, pressure-hydrated magnesium hydroxide slurry as described in U.S. Patent No. 5,487,879, the entire contents of which are incorporated herein by reference. The slurry of U.S. Patent No. 5,487,879 is modified according to the invention by adding at least about 5.0% to about 30% calcium hydroxide by weight (Mg(OH)2 basis) derived from, for example, dolime, high calcium quicklime, hydrated lime, hydrated dolime, normal hydrated dolomitic lime (Type N), highly hydrated dolomitic lime (Type S) and any related byproducts (e.g. kiln dust fines, electrostatic precipitator dust, etc.) for the purpose of enhancing reactivity when neutralizing wastewater.
The following summarizes reaction schemes consistent with the pressure hydration process of the invention:
CaO + MgO + 2H20 → Ca(OH)2 + Mg(OH)2; and CaO MgO + MgO + 3H20 -> Ca(OH)2 + 2Mg(OH)2
wherein the CaO and MgO weights are adjusted to obtain 5 to 30% Ca(OH)2 by weight (Mg(OH)2 basis).
The following summarizes a process for preparing the slurry according to this embodiment:
Cohydration under pressure of calcined natural magnesite or calcined brucite, which are both naturally occurring, or MgO derived from other sources and a calcium oxide-based compound such as high calcium quicklime or dolime as summarized below:
(a) pressure-hydrating at about 3 psig to about 150 psig, preferably at about 5 psig to about 70 psig, a mixture containing calcined natural magnesite, calcined brucite or MgO derived from other sources; at least about 5.0% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis) equivalent to high calcium quicklime or dolime; about 0.4% to about 0.6% by weight (slurry basis) polyaluminum chloride; and water to provide a pressure-hydrated modified magnesium hydroxide slurry;
(b) stabilizing the pressure-hydrated modified magnesium hydroxide slurry with about 0.02% to about 0.06% by weight (slurry basis), preferably about 0.02% by weight (slurry basis), polymer such as polyquatemary amine or alkylamine epichlorohydrin;
(c) screening the pressure-hydrated modified magnesium hydroxide slurry through, for example, 40 mesh and/or 80 mesh sieves; and
(d) deagglomerating and dispersing the Mg(OH)2 agglomerates in the pressure-hydrated modified magnesium hydroxide slurry with a homogenizer under pressure, a high speed dispersion device, a rotor stator or any other suitable means of deagglomeration and dispersion.
The pressure used in the pressure-hydration reaction is determined by the material and the construction of the reactors used and not the process. The rate of cooling can be adjusted to keep the pressure under a maximum.
The end product is a modified magnesium hydroxide slurry of "moderate quality" (i.e., about 55-65% solids content by total slurry weight, Brookfield viscosity at room temperature of 50-500 cps, pourability/flowability > 80% by weight of sample poured off after 7 days of undisturbed (unagitated) gravity settling, water separation < 1 inch (wherein, the height of water separation is measured in a standard cylindrical 8 oz. polyethylene bottle (2 in. OD x 5 3/8 in. height) and settled solids which are readily resuspendable). Resuspension characteristics were determined by the ability to resuspend any settled solids with a glass probe without difficulty.
According to another embodiment of the invention, the process involves the manufacture of a stabilized, hydrated magnesium hydroxide slurry that is hydrated at atmospheric pressures. The following summarizes a process for preparing the slurry according to this embodiment:
(a) cohydrating (1) calcined natural magnesite, calcined brucite or MgO derived from other sources; (2) a calcium oxide-based compound to provide at least about 5.0% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis); (3) water; and (4) polyaluminum chloride at about 0.4% to 0.6% by weight (slurry basis) at atmospheric pressures for about 16-24 hours to provide a hydrated modified magnesium hydroxide slurry;
(b) stabilizing the hydrated modified magnesium hydroxide slurry with about 0.02% to about 0.06% by weight (slurry basis), preferably about 0.02% by weight (slurry basis), polymer such as polyquatemary amine or alkylamine epichlorohydrin;
(c) screening the hydrated modified magnesium hydroxide slurry through, for example, 40 mesh and/or 80 mesh sieves; and
(d) deagglomerating and dispersing the Mg(OH)2 in the hydrated modified magnesium hydroxide slurry with a homogenizer under pressure, a high speed dispersion device, a rotor stator or any other suitable means of deagglomeration and dispersion.
Since the rate of hydration of the calcined magnesite, calcined brucite or MgO derived from other sources is significantly slower for this process than for the pressure hydration process, about 16-24 hours is required for the hydration reaction to take place.
A third process for preparing a modified magnesium hydroxide slurry according to the invention is described below. The process involves mixing a calcium oxide-based compound such as high calcium quicklime or dolime with diluted magnesium hydroxide slurry synthesized from any known method, e.g., the hydration methods discussed above and the brine/seawater and lime/dolime reactions disclosed in U.S. Patents Nos. 4,314,985, 5,514,357 and 3,301 ,633, the entire contents of which are incorporated by reference herein.
The following summarizes reaction schemes consistent with the process of adding a calcium oxide-based compound to a finished Mg(OH)2 product according to the invention:
CaO + Mg(OH)2 + H20 -» Ca(OH)2 + Mg(OH)2; and CaO MgO + Mg(OH)2 + 2H20 → Ca(OH)2 + 2Mg(OH)2.
In particular, the process involves:
(a) diluting magnesium hydroxide slurry produced from e.g., a brine/seawater process from about 60% to about 49% magnesium hydroxide solids, for instance, and adding high calcium quicklime to yield 30% Ca(OH)2 by weight (Mg(OH)2 basis) or from about 60% to about 58% Mg(OH)2 solids, for instance, and adding high calcium quicklime to yield 5% Ca(OH)2 by weight (Mg(OH)2 basis).
(b) adding at least about 5% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis) in, for example, the form of high calcium quicklime or dolime, to the diluted slurry to produce a modified magnesium hydroxide slurry;
(c) hydrating the high calcium quicklime or dolime contained in the modified magnesium hydroxide slurry by any suitable means of agitation;
(d) stabilizing the modified magnesium hydroxide slurry with at least about 0.02% to about 0.06% by weight
(slurry basis) polymer; and
(e) deagglomerating and dispersing the Mg(OH)2 in the slurry with a homogenizer under pressure, a high speed dispersion device, a rotor stator, or any other suitable means of deagglomeration and dispersion.
Additional reactivity of the modified magnesium hydroxide slurry of the invention can be achieved by adding sodium hydroxide to the modified magnesium hydroxide slurry of the invention. As mentioned previously, NaOH solubilizes quickly in water. Thus, adding a percentage of NaOH 1-10% by weight to magnesium hydroxide slurry would result in an increased rate of reactivity and a higher endpoint pH.
By using the modified magnesium hydroxide slurry according to the invention, wastewater, and in particular wastewater containing high metal concentrations, can be treated more economically, since the modified magnesium hydroxide slurry (1) reduces the amount of slurry required to reach a target pH when compared to standard magnesium hydroxide slurry and (2) reduces the amount of sludge generated for disposal during metal precipitation when compared to lime or caustic soda.
Referring to FIG. 1 , a typical wastewater treatment system 1 is shown. In such a system, transition or heavy metals (not shown) are removed via hydroxide precipitation. The waste stream 2 is separated from the return sludge 9, which is discharged as waste sludge 10 for further processing or disposal, and neutralized with an alkali 3 such as magnesium hydroxide slurry, lime or caustic soda in a neutralization tank 4 for a specified amount of time (i.e., retention time). The retention time of the system 1 is built into the design of the treatment system 1 and depends on the wastewater flow rate and the size of the neutralization tank 4. A pH probe 5 is immersed in the neutralization tank 4 to monitor the rate of neutralization and indicate when more alkali 3 is needed to reach a target pH. The target pH usually corresponds to the pH of the minimum solubility for a particular metal hydroxide in water. For example, manganese has a minimum solubility in water at a pH >8.5. Once the target pH is reached, most of the soluble metal present in the waste stream is precipitated out as a metal hydroxide. A polymer 6 (e.g., flocculent) may be added to the wastestream 2 and the waststream 2 may be flocculated in a flocculator 11 in order to agglomerate smaller metal hydroxide particles into larger particles for faster settling. The settled metal hydroxide particles (not shown) are collected at the bottom of a clarifier 7 as waste sludge 10 for further processing or disposal. The clarified filtrate (not shown) is then properly discharged as effluent 8.
Table 1 below shows the particle sizes of ferric hydroxide sludge generated after neutralization of an acidic solution containing iron using caustic soda, hydrated lime, standard magnesium hydroxide slurry, and the modified magnesium hydroxide slurry of the invention. Table 1 exemplifies the sludge benefits of the modified magnesium hydroxide slurry over lime and caustic soda. As Table 1 shows, the standard and the modified magnesium hydroxide slurries produce sludge crystals having larger particles than lime and caustic soda. The resulting sludge is more dense and compact and thus, results in lower sludge volume. The larger sludge particle produced by standard Mg(OH)2 slurry and the modified Mg(OH)2 slurry eliminates the need to recycle sludge as frequently done in lime systems to build larger sludge particles for better clarification and filtration. Additional equipment for recycling sludge is therefore eliminated which results in reduced capital costs.
TABLE 1
Resulting Particle Size of Ferric Hydroxide Sludge After Neutralization of a Ferrous Sulfate Solution Using Various Alkalis
Figure imgf000019_0001
* Measured by Micromeritics Sedigrap 5100
1 MagneClear™ 58 Magnesium Hydroxide Slurry
2 10.6% by wt. Ca(OH)2 Modified Magnesium Hydroxide Slurry
As discussed above, when treating metallic ions, magnesium hydroxide forms metallic hydroxide sludge, which has a higher density, a higher solids content and a smaller volume than sludge produced from lime or caustic soda due to a larger metallic hydroxide particle size as compared to smaller metallic hydroxides made from lime or caustic soda. Lime and caustic soda cause flash
(i.e., immediate) precipitation of the metallic hydroxides. This results in extremely fine particles that agglomerate and become gelatinous and difficult to separate either by filtration or settling. As a result, the reduction in sludge volume reduces the overall costs associated with sludge disposal. Thus, the use of the modified magnesium hydroxide slurry of the invention to treat wastewater is advantageous over the use of lime and caustic soda. In addition, unlike lime and caustic soda, the modified magnesium hydroxide slurry of the invention buffers to a lower pH and thus provides better pH control. Adding up to about 30% Ca(OH)2 by weight
(Mg(OH)2 basis) into magnesium hydroxide slurry does not adversely affect sludge volume or density, while significantly reducing the reaction time when compared to standard magnesium hydroxide slurry. Thus, use of the modified magnesium hydroxide slurry of the invention is also advantageous over use of standard magnesium hydroxide slurry.
The modified magnesium hydroxide slurry of the invention enhances reactivity in wastewater neutralization systems, particularly in the presence of high amounts of transition and heavy metals such as Fe+3, Mn+2, Cu+2, Cr+3, Ni+2, etc. As many wastewater treatment plants are designed for fast reacting alkalis such as lime or caustic soda, the plants generally have limited retention time in neutralization tanks and thus frequently permit too short a reaction time for standard magnesium hydroxide slurries to be effective. The alkalinity of the treated water often is too low for manganese and other metals to precipitate within the allotted reaction time. Adding small quantities of calcium hydroxide to the magnesium hydroxide slurry causes the slurry to react faster without jeopardizing the advantage of more compact sludge compared to the gelatinous and voluminous sludge produced by lime or caustic soda.
Another feature of utilizing the modified magnesium hydroxide slurry in accordance with the invention in treating wastewater containing high amounts of transition and heavy metals is the ability to achieve a higher endpoint pH, while requiring less amount of slurry on a weight % basis than standard magnesium hydroxide slurry. As pointed out in the examples below, by achieving a higher endpoint pH, improved metal removal can occur, particularly with those metals having minimum solubilities at pHs >8.5.
Other applications for modified magnesium hydroxide slurry include:
(1) treating acidic waste streams such as those containing hydrofluoric and phosphoric acids;
(2) treating waste streams containing anions such as fluorides, phosphates, and arsenic; and
(3) removing SOx during scrubbing applications (standard Mg(OH)2 slurry has been successfully applied to scrubbing applications for removal of SOx via sulfite precipitation). Example 1
The following example demonstrates the synthesis of the modified magnesium hydroxide slurry using pressure hydration discussed above.
A laboratory autoclave was charged with calcined natural magnesite, high calcium quicklime or dolime, water, and polyaluminum chloride. Various ratios of calcined natural magnesite to high calcium quicklime or dolime were used to provide a minimum of about 5.0% Ca(OH)2 by weight (Mg(OH)2 basis) and a maximum of about 30% Ca(OH)2 by weight (Mg(OH)2 basis). Approximately 0.4 to 0.6% by weight (slurry basis) of polyaluminum chloride was added to increase the degree of hydration of calcined magnesite and reduce the viscosity of the resulting slurry. The above ingredients were then pressure hydrated in the autoclave at 324°F and 100 psig for 10 minutes and cooled to ambient temperatures. A total residence time of 225 minutes in the autoclave including heat-up and cool-down was employed. However, the amount of time may vary with the system and equipment used.
A suitable polymer for use with this invention, either alkylamine epichlorohydrin polymer or polyquatemary amine was added in an amount equal to 0.02 - 0.06% by weight (slurry basis) to the pressure hydrated slurry while blending on low speed with a Shar dispersion mixer. The slurry was then screened through 40 mesh (i.e., 420 microns) and 80 mesh (i.e., 177 microns) sieves and processed through an APV Gaulin 15MR-8TA laboratory homogenizer at pressures ranging from about 1000 to 2500 psig and at two or three passes to provide better particle dispersion and a more flowable product. Stability tests on the modified magnesium hydroxide slurry showed encouraging pour results, i.e., >80% by weight of slurry poured after 7 days of undisturbed gravity settling and < 1 inch water separation, and sufficient stability for transportation by truckloads.
Several modified magnesium hydroxide slurry formulations were developed, stabilized, and tested. Table 2 below shows modified slurry formulations and their degree of stability as measured in % pour, which was determined according to the following: weight of slurry poured off from an 8 oz. bottle after 30 seconds X 100 total weight of slurry
TABLE 2 DEGREE OF STABILITY
Figure imgf000022_0001
* Measured at 50% solids
1 u = not homogenized
2 h = homogenized
As can be seen in Table 2, based on a 14-Day Pour, 4.5% Ca(OH)2, 5% Ca(OH)2 and 9% Ca(OH)2 by weight (Mg(OH)2 basis) exhibited good stability and resuspension characteristics.
Example 2
Depending upon the type of wastewater treated, standard magnesium hydroxide slurry typically buffers at a pH of about 8.5-9.0, even with an overaddition of Mg(OH)2. Using a modified magnesium hydroxide slurry according to the invention, a higher buffering pH can be achieved, which in turn results in effective metal removal for those metals requiring a higher target pH. FIG. 2 shows comparative buffering capacities between MagneClear™58 Magnesium Hydroxide Slurry and the modified magnesium hydroxide slurry prepared according to Example 1 of the invention containing 10.6% Ca(OH)2 by weight (Mg(OH)2 basis) for a 1N solution of hydrochloric acid. Both alkali dosages were based on an overaddition of 200% excess the stoichiometric amount (with respect to a NaOH titration of the acid solution). After two hours, MagneClear™58 Magnesium Hydroxide Slurry buffered at a pH of 8.67, wherein the modified magnesium hydroxide slurry of the invention at 10.6% Ca(OH)2 by weight (Mg(OH)2 basis) reached a peak pH of 9.11 and buffered at a pH of 8.96. In most cases, this higher buffering capacity will allow the modified magnesium hydroxide slurry to achieve effective metal removal above pH 8.5.
Example 3
Preliminary neutralization evaluations were performed using a sample of coal pile leachate containing high iron (2919 ppm) and manganese (34 ppm) concentrations. A four hour retention time was employed for 200 mL jar tests. Two modified slurry formulations were tested. FIG. 3 illustrates the rate of neutralization for 90% Ca(OH)2 by weight (hydroxide basis) hydrated lime slurry, 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) MagneClear™58 Magnesium Hydroxide Slurry and 5% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry.
Based on a 1 N NaOH titration for this wastewater sample, 20% excess the stoichiometric amount of hydrated lime slurry, 40% excess the stoichiometric amount of MagneClear™58, and 15% excess the stoichiometric amount of 5% Ca(OH)2 by weight (Mg(OH)2 basis) slurry were required to raise the pH of the untreated coal pile leachate sample from pH 2.3 to 7.5-8.5.
From FIG. 3, the untreated wastewater sample was raised to a pH of 8.3 with hydrated lime slurry and to a pH of 8.0 with MagneClear™58. This graph shows that the modified magnesium hydroxide slurry, which incorporates a percentage of dolime, (i.e., 10% dolime by weight (MgO basis)) has an increased rate of neutralization when compared to MagneClear™58. Only 15% excess the stoichiometric amount of 5% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry was required to elevate the pH of the wastewater to pH 8.0, wherein 40% excess the stoichiometric amount of MagneClear™58 was required to elevate the pH of the wastewater to pH 8.0. Thus, less of a percent excess on a slurry weight basis is required by the modified magnesium hydroxide slurry than MagneClear™58 to reach the same endpoint pH. Table 3 shows the results of the neutralization tests:
TABLE 3
NOTES: 1 ) All results are in ppm.
2) NT = not tested
3) Slurries are expressed as % Ca(OH)2 by weight on a hydroxide basis
4) Hydrated lime slurry
5) MagneClear™ 58 Magnesium Hydroxide Slurry
6) 10% Dolime by weight slurry.
As the results in Table 3 show, iron removal to a level below 1 ppm was achieved by each of the alkalis. Manganese removal was only reduced to a level of 2.5 ppm using hydrated lime slurry. Since manganese removal is typically achieved at a pH >8.5, treatment with MagneClear™58 and 5% Ca(OH)2 by weight (Mg(OH)2 basis) slurry were unable to remove manganese to low concentrations. The MagneClear™58 and the modified magnesium hydroxide slurries, however, were effective in reducing the amount of sludge volume produced by 40% and 72% respectively when compared to hydrated lime slurry.
Example 4
FIGS. 4 and 5, respectively, compare neutralization rates on a stoichiometric basis and a 50% excess the stoichiometric basis (with respect to a NaOH titration on a representative wastewater sample) between modified magnesium hydroxide slurry containing 10.6% Ca(OH)2 by weight (Mg(OH)2 basis) and two standard magnesium hydroxide slurries, FloMag®H, a synthetic slurry produced from brine and dolime having a typical Ca(OH)2 content of about 0.92% by weight (Mg(OH)2 basis), and MagneClear™58, a slurry produced from pressure hydrating naturally occurring calcined magnesite and water having a typical Ca(OH)2 content of about 1.98% by weight (Mg(OH)2 basis). The wastewater in this case is ash refuse leachate containing approximately 1200 ppm ferric iron and 48 ppm manganese. The graphs in FIGS. 4 and 5 show that the modified magnesium hydroxide slurry of the invention neutralizes the leachate containing a high iron concentration faster than and achieves an endpoint pH after 150 minutes of retention time higher than both of the standard magnesium hydroxide slurries.
Table 4 shows (1) ferric iron concentration, (2) manganese concentration and (3) endpoint pH after treatment with each alkali.
TABLE 4
Comparison of Metal Removal Efficiency Using Various Alkalis for Treatment of Ash Refuse Leachate
Figure imgf000026_0001
These results show that based on a 50% excess the stoichiometric amount of each alkali, the modified magnesium hydroxide slurry improved manganese removal by reducing Mn+2 from 48 ppm to 1.1 ppm due to the higher endpoint pH. The standard magnesium hydroxide slurries were ineffective in lowering manganese to low concentrations as shown by the 7 ppm and 9.2 ppm manganese concentrations for FloMag®H and MagneClear™58, respectively.
Example 5
The following example demonstrates the effectiveness of the modified magnesium hydroxide slurry prepared according to Example 1.
Neutralization tests were run on an ash refuse leachate wastewater sample containing high iron and manganese concentrations (i.e., 1200 ppm Fe+3 and 48 ppm Mn+2). The following alkalis were utilized for neutralization and metal precipitation: (1) a modified magnesium hydroxide slurry containing 10.6% Ca(OH)2 by weight (Mg(OH)2 basis), (2) 0.92% Ca(OH)2 by weight (Mg(OH)2 basis) (FloMag®H), (3) 1.98% Ca(OH)2 by weight (Mg(OH)2 basis) (MagneClear™58), (4) 96.66% Ca(OH)2 by weight (hydroxide basis) (hydrated lime slurry) and (5) 55.16% Ca(OH)2 by weight (Mg(OH)2 basis) (hydrated dolime slurry).
As seen in FIG. 6, the modified magnesium hydroxide slurry increased the rate of neutralization and reached a higher endpoint pH of 8.48 versus 7.9 for FloMag®H and 8.08 for MagneClear™58 Magnesium Hydroxide Slurries. Thus, less modified magnesium hydroxide slurry was required (by weight) than standard magnesium hydroxide slurry to reach the same target pH, and improved manganese removal was achieved due to the higher reactivity (see Table 5). In addition, the modified magnesium hydroxide slurry reduced the amount of sludge volume generated by 78% and 44% when compared to treatment with hydrated lime slurry and hydrated dolime slurry, respectively. Table 5 shows (1) ferric ion concentration, (2) manganese concentration, (3) sludge volume and (4) endpoint pH after treatment with each alkali.
TABLE 5 Comparison of Metal Removal Efficiency and Sludge Volume Using Various Alkalis for Treatment of Ash Refuse Leachate
Figure imgf000028_0001
1 Modified Magnesium Hydroxide Slurry
2,3 Magnesium Hydroxide Slurries
4 Hydrated Lime Slurry @ 10% solids
5 Hydrated CaO.MgO Slurry @ 20% solids
6 pH after 150 minutes retention time
Example 6
Neutralization tests on a fresh sample of coal pile leachate were conducted to further compare slurry reactivity. Alkali was added immediately to the untreated coal pile leachate sample. After 80 minutes of rapid agitation on a stir plate, air was bubbled through the beaker to simulate aeration in the second stage reactor (neutralization tank). In this treatment system, alkali is added in the first neutralization tank while aeration is added in the second neutralization tank. This aeration along with rapid agitation were applied for an additional 138 minutes for a total neutralization time of 218 minutes. Based on an initial titration with 1 N NaOH, approximately 45% excess and 30% excess the stoichiometric amount of MagneClear™58 Magnesium Hydroxide Slurry, 30% excess the stoichiometric amount of hydrated lime slurry, 5% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry, 16% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry and 9% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry were required to raise the pH of the raw wastewater from pH 2.6 to pH 8.0-8.5. FIG. 7 illustrates the rate of neutralization for this particular wastewater sample. Improved reactivity can be seen using the modified magnesium hydroxide slurry formulations which incorporate dolime or high calcium quicklime. Fifteen percent less, i.e., 30% excess (on slurry weight basis) of the modified magnesium hydroxide slurry blends is required to reach the same pH as a 45% excess the stoichiometric amount of MagneClear™58.
Table 6 summarizes the results:
TABLE 6
Figure imgf000030_0001
NOTES:
1 ) All results are in ppm.
2) * pH of clarified wastewater after treatment.
3) ** Two week settling period.
4) Slurries are expressed as % Ca(OH)2 by wt. on a hydroxide basis.
5) Dolime and High Calcium Quicklime (i.e., Hi-Cal Lime) percentages are on a MgO basis.
6) Hydrated lime slurry.
7) MagneClear™ 58 Mg(OH)2 slurry.
8) 10% Dolime by weight slurry.
9) 10% HiCal lime by weight slurry.
10) 30% Dolime by weight slurry.
As can be seen from these results, MagneClear™58 and each of the modified magnesium hydroxide slurry formulations achieve good iron removal resulting in less than 1.0 ppm Fe+3 content. The modified magnesium hydroxide slurries of the invention exhibited improved manganese removal. The 5% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry reduced the manganese from 44 ppm to 1.9 ppm; the 9% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry proved to be even more effective in reducing manganese to 1.4 ppm; and the 16% Ca(OH)2 by weight (Mg(OH)2 basis) modified magnesium hydroxide slurry achieved 0.04 ppm manganese.
As demonstrated in Example 5, a significant sludge volume reduction is achieved using MagneClear™58 and the modified magnesium hydroxide slurry formulations according to the invention to neutralize coal pile leachate.
Table 7 below shows the % reduction in sludge volume for each slurry when compared to hydrated lime slurry:
TABLE 7
% Sludge Volume Reduction Compared to Hydrated Lime Slurry for Treatment of Coal Pile Leachate
Figure imgf000031_0001
These tests show that the modified magnesium hydroxide slurries of the invention have (1) increased rates of neutralization, (2) improved manganese removal, (3) reduction in alkali requirement on a slurry weight basis when compared to MagneClear™58 Magnesium Hydroxide Slurry, and (4) significant sludge volume reduction when compared to hydrated lime slurry.
The embodiments described above provide a number of significant advantages, including the use of a modified magnesium hydroxide slurry that allows a more rapid and effective treatment of wastewater to be conducted using current wastewater treatment systems.
Of course, it should be understood that a wide range of changes and modifications can be made to the preferred embodiment described above. It is therefore intended that the foregoing detailed description be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.

Claims

What is claimed is:
1. A process for preparing a modified magnesium hydroxide slurry having high calcium hydroxide content comprising: adding a calcium oxide-based compound during a magnesium hydroxide slurry manufacturing process to obtain a modified magnesium hydroxide slurry comprising about 5% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis).
2. The process according to claim 1 , wherein said calcium oxide-based compound is added to obtain a modified magnesium hydroxide slurry comprising about 10% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis).
3. The process according to claim 1 comprising: hydrating a mixture containing calcined natural magnesite, calcined brucite or MgO derived from other sources with a calcium oxide-based compound and water.
4. The process according to claim 3, wherein said mixture is hydrated at atmospheric pressures.
5. The process according to claim 3, wherein said mixture is hydrated at a pressure of about 3 psig to about 150 psig.
6. The process according to claim 3, wherein said mixture is hydrated at a pressure of about 5 psig to about 70 psig.
7. The process according to claim 3, wherein said mixture further comprises chloride ion.
8. The process according to claim 7, wherein polyaluminum chloride is a source of said chloride ion.
9. The process according to claim 8, wherein said mixture comprises about 0.4% to about 0.6% polyaluminum chloride.
10. The process according to claim 1, wherein said mixture further comprises polymer.
11. The process according to claim 10, wherein said polymer is selected from the group consisting of polyquatemary amine polymer and alkylamine epichlorohydrin polymer.
12. The process according to claim 11 , wherein said polymer is present in an amount of about 0.02% to about 0.06% by weight (slurry basis).
13. The process according to claim 1 , wherein said magnesium hydroxide slurry is made from a member selected from the group consisting of seawater, brine and magnesia-bearing ore.
14. The process according to claim 1 , wherein said calcium oxide-based compound is selected from the group consisting of high calcium quicklime, dolomitic quicklime, slaked lime, normal hydrated dolomitic lime - Type N, highly hydrated dolomitic lime - Type S, kiln dust fines and ESP dust fines.
15. A modified magnesium hydroxide slurry comprising at least about 5% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis).
16. A process for treating wastewater comprising: treating said wastewater with a modified magnesium hydroxide slurry that comprises at least about 5% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis), wherein said wastewater comprises waste selected from the group consisting of organic and inorganic acids, heavy and transition metals and anions.
17. The process according to claim 16, wherein said wastewater comprises heavy and/or transition metals selected from the group consisting of Fe+3, Fe+2, Ni+2, Ag+2, Mn+2, Co, Au, Ti, Mo, Pt, Cd+2, Al+3, Cu+2, Zn+2, Pb+2, Cr+3, Sb, Hg, Sn, and Bi.
18. The process according to claim 16, wherein said wastewater comprises heavy and/or transition metals at concentrations greater than or equal to 100 ppm.
19. The process according to claim 17, wherein said wastewater comprises Fe+3 at concentrations greater than or equal to 100 ppm.
20. The process according to claim 16, wherein said wastewater comprises organic and/or inorganic acids selected from the group consisting of acetic acid, hydrofluoric acid, phosphoric acid, H2S04, HCI, HBr, H2S03, HN03 and citric acid.
21. The process according to claim 16, wherein said wastewater comprises anions selected from the group consisting of as fluorides, phosphates and arsenic.
22. A process for preparing a modified magnesium hydroxide slurry having high calcium hydroxide content comprising: pressure-hydrating a mixture containing a calcium-oxide-based compound to provide at least about 5% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis), polyaluminum chloride, and water to provide a pressure-hydrated modified magnesium hydroxide slurry; stabilizing the pressure-hydrated modified magnesium hydroxide slurry with polymer selected from the group consisting of polyquatemary amine and alkylamine epichlorohydrin; screening the pressure-hydrated modified magnesium hydroxide slurry; and deagglomerating and dispersing the Mg(OH)2 agglomerates in the pressure- hydrated modified magnesium hydroxide slurry.
23. A process for preparing a modified magnesium hydroxide slurry having high calcium hydroxide content comprising: cohydrating calcined natural magnesite, calcined brucite or MgO derived from other sources, a calcium-oxide-based compound to provide at least about 5% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis), water and polyaluminum chloride at atmospheric pressures to provide a hydrated modified magnesium hydroxide slurry; stabilizing the hydrated modified magnesium hydroxide slurry with polymer; screening the hydrated modified magnesium hydroxide slurry; and deagglomerating and dispersing the Mg(OH)2 in the hydrated modified magnesium hydroxide slurry.
24. A process for preparing a modified magnesium hydroxide slurry having high calcium hydroxide content comprising: mixing a calcium oxide-based compound such as high calcium quicklime or dolime with magnesium hydroxide slurry, wherein said modified magnesium hydroxide slurry comprises at least about 5% to about 30% Ca(OH)2 by weight (Mg(OH)2 basis).
25. The process according to claim 16, wherein said wastewater comprises transition and/or heavy metals, wherein said modified magnesium hydroxide slurry reacts with said transition and/or heavy metals to produce metal hydroxide particles having a mean particle size of greater than or equal to 8 urn, and wherein said particles having a mean particle size greater than or equal to 8 urn provide for better clarification and filtration of said transition and/or heavy metals from said wastewater.
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